This invention relates to a two-wire system including a power source, a signal receiving section and a detection section in which system, the receiving section is connected to the power source and has a relay in connection with a large number of parallel detection circuits for detecting critical changes and with a circuit for protecting the relay.
In the two-wire system including the signal receiving section or circuit supplied with electrical power from the power source and connected to a great many parallel detection circuits at the ends thereof in parallel therewith a relay is employed in this receiving or receiver circuit as means for receiving the detecting operation or detection signal of the detection circuit in the receiver circuit. FIGS. 1 and 2 show examples of this two-wire system.
The two-wire system shown in FIG. 1 has a D.C. power circuit which includes a power transformer 1, four diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 connected in bridge form to the output of the transformer 1 for rectifying an A.C. current from the transformer 1, and a smoothing condenser SC connected to the output of the bridge diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 for smoothing the pulsating current from the bridge diodes to produce a D.C. current at first terminals 2a and 2b; a receiver circuit which includes a relay 3 connected between the two terminals 2a and 2b of the power circuit to produce an output at second terminals 2c and 2d thereof; and a detection circuit which includes sensors P.sub.1 and P.sub.2 connected between third terminals 2e and 2f thereof connected respectively to the second terminals 2c and 2d. When any of the sensors P.sub.1 and P.sub.2 detects to close a circuit including the D.C. power circuit and the relay 3, a D.C. current will flow from the D.C. power circuit through the relay 3 and the detected sensor to energize the relay 3 to cause the contacts S.sub.1 and S.sub.2 of the relay 3 to be closed to illuminate a fire indicator lamp L.
The two-wire system shown in FIG. 2 employs a receiver circuit which includes a switching semiconductor or transistor Q in addition to a relay 3 for further accurately controlling the operation of the relay 3, wherein the other parts and components are the same as those shown in FIG. 1 and the description thereof is omitted. A resistor R.sub.2 is connected in the receiver circuit for voltage-controlling the operation of the transistor Q in such a manner that the transistor Q is enabled and conducts when the voltage difference across the resistor R.sub.2 reaches a predetermined voltage value.
As shown in FIGS. 4a and 4b, the detection circuit also includes a condenser C.sub.1 having a relatively large electric capacitance for compensating the load voltage of the sensors P.sub.1 and P.sub.2 connected in parallel with the detection circuit, required for the ordinary and detecting operations of the senors P.sub.1 and P.sub.2 resulting in occurrence of the following new problems. That is, if the number of sensors is increased in parallel with the two-wire system including the receiver circuit and the detection circuit in order to raise the sensing or detecting capacity of the detection circuit, the number of the voltage compensating capacitors C.sub.1 are accordingly increased. As a result, the current capacity of the power source for supplying a load current to the sensors of the detection circuit is also increased, so that the duration time of the instantaneous transient current flowing through the circuit upon activation of the power source is lengthened with the result that the relay 3 may sometimes operate to close its contacts within this transient time. Accordingly, the relay 3 must be protected from such operation during this transient time until this transient current is charged in the capacitor.